The goal of this research is to delineate the evolutionary potential and gene structure-biological function relationships of bunyaviruses in their arthropod vectors at the organism, organ, and cell level. The molecular basis of bunyavirus development in and transmission by vectors will be defined using parental and defined clone RNA segment reassortant viruses. Determination of bunyavirus gene contributions to vector-virus interactions should permit a better understanding of factors operative in maintenance of arbovirus cycles in nature. The potential for RNA segment reassortment and generation of new reassortant viruses will be determined in dually infected vectors. In addition, the potential for spontaneous bunyavirus mutation in persistently (transovarially) infected vectors will be investigated. Members of the Bunyaviridae are of considerable medical and veterinary importance. Determination of the evolutionary potential of the viruses by RNA segment reassortment and by spontaneous mutation combined with comparative pathogenesis studies of parent, reassortant, and mutant viruses, might indicate which way the epidemic potential of the viruses would evolve.